Is Arctic ice thinning?

CryoSat's detailed data have been used to generate this map of sea-ice thickness in the Arctic with data from January and February. Thanks to CryoSat's orbit, ice thickness close to the North Pole can be seen for the first time.

Scientists have long used satellite imagery to illustrate the shrinking extent of the Arctic sea ice. Now they've got satellite data that will provide regular updates on whether the ice is getting thinner as well.

The first ice thickness map from the European Space Agency's CryoSat spacecraft was released Tuesday at an air show in Paris. It was compiled with data collected in January and February.

The map shows, for example, the ice is thickest near the North Pole and off the coasts of Greenland and northeastern Canada. It thins as it stretches out towards Alaska and Russia.

Scientists expect the imagery to complement studies that show the Arctic sea ice extent is shrinking. This winter, for example, U.S. scientists reported the sea ice extent was among the smallest ever seen.

These warnings are based on models and observations of the sea ice extent — that is how much of the Arctic Ocean the ice covers. For a more robust understanding, scientists also need to know how thick the ice is.

The winds could, for example, push the ice out of one area but pile it up in another. This would mean the ice extent had diminished, but the volume remained the same.

CryoSat measures the thickness of the ice, providing a 3-D view, Walt Meier, a research scientist at the National Snow and Ice Data Center in Boulder, Colorado, told me today.

"Looking at the extent, we are just looking down at the surface, sort of the facade of the ice cover and you don't know exactly what it looks like underneath," he said. CryoSat, he added, will provide regularly updated pictures on the volume of sea ice.

Scientists have previously obtained ice thickness measurements, but only a few times a year with instruments such as NASA's IceSat, Meier noted. The new satellite provides continuous data. "It can show us how the ice changes seasonally and from year to year," he said.

The satellite obtains thickness measurements with a technique that bounces radar waves off the ice and the water in cracks which separate the ice floes. A calculation allows them to determine the sea ice thickness above the water.

Of course, about 90 percent of sea ice is actually underwater, "but if you measure the 10 percent above and know roughly what the density (of the ice) is, which tells you how much is above versus below, then you can calculate the total thickness," Meier said.

CPOM / UCL / ESA / Planetary Visions

For the first time, data from ESA's CryoSat mission have been used to map the height of the ice sheet that blankets Antarctica. CryoSat's ability to map the edges of the ice sheet is demonstrated by the detail that can be seen of the flow from east Antarctica onto the Ronne-Filchner ice shelf in the west. The outer white circle represents the limits of earlier missions and the inner circle shows that CryoSat is collecting data up 88° latitude.

For now, the thickness data shows the ice thickness in January and February. But in coming months more maps will be released and, over time, that allow scientists to see year-to-year changes in ice thickness.

"The data are exceptionally detailed and considerably better than the mission specification," the ESA writes in a news release. "They even show lineations in the central Arctic that reflect the ice's response to wind stress."

Meier cautioned, however, that the data is "fresh off the presses," more a proof of concept that the satellite can see sea ice and measure its thickness. "There are still a lot of things to work out ... it is too early to put a lot of stock in the absolute numbers."

In addition, the researchers have created a new map of Antarctica showing the height of the ice extent there. The data here is also preliminary, but shows CryoSat's ability to map the edges of the ice sheet in detail.

Understanding how ice sheets are changing at edges of Antarctica and Greenland is key, since change is happening fastest at the edges.